1. Field of the Invention
The present invention relates to a keep-alive backup power supply for a volatile electronic memory. More particularly, the present invention relates to an energy management circuit for maintaining a solid-state electronic memory in an active condition according to predefined circuit parameters and in response to various modes of circuit operation.
2. Description of the Prior Art
Variable or programmed data are stored in electronic devices in solid-state electronic memory arrays. Because such memory arrays are composed of volatile devices, a constant source of power must be supplied to the memory arrays to retain data entered therein in the memory. It is well known to provide a backup power source for such memory arrays to retain data in the event of a main power source failure. In this way, important information in the form of data is maintained intact during power outages or other power source interruptions.
One common electronic device incorporating solid-state memory is the telephone. Telephone numbers, access codes, etc. are now stored as data in electronic memory arrays. FIG. 1 shows a typical prior art telephone installation 10 including a first telephone circuit 11a and another of any number of extension telephone circuits 11n.
Telephone circuit 11a is coupled to the telephone line through an on-hook/off-hook switch 12. The telephone circuit provides a power bus Udd for powering dialing devices and other such devices which incorporate memories, such as memory device 13. When the phone is off-hook (in use), power from the line is connected through the telephone circuit 11a over power bus Udd to the dialing or other memory device 13. In this way, the device is kept active and data therein is maintained intact. A capacitor C1 may be included for accumulating a "keep alive" charge during the off-hook interval or alternately, a battery 14 may be provided as a backup memory power source. When the phone is placed on-hook (not in use), a trickle charge of no more than 5 microamperes is provided through resistor R1 to trickle charge capacitor C1 or battery 14.
A disadvantage of the prior art memory keep alive arrangement is that the amount of current provided to trickle charge the capacitor is usually insufficient to maintain the capacitor with an adequate charge to keep device 13 active over a long period of time. This problem is exacerbated if another extension, such as telephone circuit 11n is used. Such use draws power from the line and therefore provides less current through resistor R1 to trickle charge the capacitor.
The addition of a very large capacitor or battery for storing larger amounts of current during the off-hook condition provides no solution to the problem of keeping a solid state memory alive during the on-hook condition. The amount of trickle charge current provided by resistor R1 is too insignificant to fully charge the large capacitor or a rechargeable battery. Accordingly, loss of memory contents in such device is a significant problem which has heretofore remained unsolved in the art. The most common, though far from ideal, solution to this problem is to provide a backup battery which requires routine replacement. Such a battery adds to the expense of the device and yet it is still unable to keep the memory contents from being lost during equipment service and battery replacement intervals.